Exhaust gas purification

ABSTRACT

This invention is concerned with a process for the effective removal of carbon monoxide hydrocarbons and the oxides of nitrogen from the exhaust stream of mobile internal combustion engines. This purification is accomplished by passing the exhaust gas from an engine operating at near the stoichiometric fuel-air ratio through a high-temperature catalyst. This catalyst is normally a noble metal catalyst. The exhaust gas is then cooled to a temperature in the vicinity of 700* F., ammonia is added and the ammoniated stream is passed over a second catalyst which can be either a base metal catalyst or a noble metal catalyst.

United States Patent 23/2 E, 23/288 F Fol-all, FOln 3/16 60I30;23/2 E288R23 TO ANALYTlCAL co a He CATALYST Inventors Jen-nua- 156-1leieraoencled W; UNITED STATE PATENTS grf f' 1,s91.11o 12/1932 Note60/30 P n 3.142.150 7/l964 60/30 5:12;"... M $272,884 l0Il966Nonnenrnacher 23/2 8 [73] Aaaignee lerdlletercaqaoy Y I I m 3,449,063 6/I969 Grifl'lng 23/2 E Primary Examiner- Douglas Hart Attorneys-John R.Faulkner and Thomas H. Oster aasnAcnmmmnoni-mmam-mu; Y the eflectiveremoval of carbon monoxide hydrocarbon and the oxides of nitrogen fromthe exhaust streant' ofnsobile interf g: nal combustion engines, Thispurification ia aceornpliahed by s mm passing the exhaust gas from anengine operating at near the [$2] 0.8. 60/30, stoichiometric fuel-airratio through a high-temperature catalyst. This catalyst is normally anoble metal catalyst. The exhaust gas is then cooled to a temperature inthe vicinity of 700' F., ammonia is added and the ammoniated atreal ipassed over a second catalyst which can beeltlser a-h-e metal catalystor a noble metal catalyst.

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EXHAUST COOLER PATENIED min 71911 saw 3 or 3 T\. M395 oz 2 A X fioi q x8 AIR TO FUEL WEIGHT RATIO INVENTORS R @m n wom MHz/ m A z W EXHAUST GASPURIFICATION 10, I969 to Griffing et al. This patent is made a portionof this record and is incorporated as a portion of this document in totoby reference. The purposes of the Griffing et al. patent are succinctlyset forth in lines 20 to 26 of column I as follows:

This invention relates to a method of reducing the unburned hydrocarbon,carbon monoxide and oxides of nitrogen content of the exhaust gas ofinternal combustion engines. In particular, this invention relates to amethod of reducing the noxious components of exhaust gas by contactingthe exhaust gas together with oxygen, ammonia and a copper-containingcatalyst."

This invention also relies upon the ability of ammonia to destroy oxidesof nitrogen in exhaust gas in the presence of oxygen in limited amountsby passing an ammoniated exhaust stream over a catalyst at comparativelylow temperatures.

This invention differs from that of Griffings et al. in that thecatalytic beneficiation of the exhaust stream is accomplished in twodistinct steps, one operating a high temperature and the second at alower and controlled temperature. The initial purification step iscarried out directly upon the very hot exhaust gas as it leaves theengine and is intended to reduce the carbon monoxide and hydrocarboncontent of the exhaust stream. No ammonia is added at this step. Thesomewhat purified exhaust stream emerging from the initial step iscooled and treated with a small addition of ammonia. This cooled andammoniated exhaust stream is then passed over a second catalyst bed toproduce a very acceptable effluent.

To facilitate a comprehension of this invention the followin g drawingshave been prepared in which FIG. I is a schematic drawing of theapparatus employed in the execution of this invention, and

FIG. 2 is a graphical showing of the effect of temperature upon oxide ofnitrogen destruction, and

FIG. 3 is a graphical showing of the efficiency of oxides of nitrogenremoval at various levels of ammonia operating at 850 F., and

FIG. 4 is a graphical showing of the chemical constituents of a normallycruising vehicle engine as a function of the air to fuel weight ratio.

FIG. 1 depicts schematically a system which has been employed for thereduction of carbon monoxide, hydrocarbons and oxides of nitrogenoriginating in one bank of cylinders ofa conventional V-8 vehicularengine. Attention is invited to the fact that the hot exhaust stream isled directly toa first catalyst for the removal of carbon monoxide andhydrocarbons. This catalyst operates upon the untreated exhaust streamand is capable of operating over a very wide range of temperatures, andis in fact limited only in .that the stream must be heated sufficientlyto be operative and that sufiicient oxygen be available. It is capableof operating at any elevated temperature encountered in engine exhaust.The exhaust stream from this first catalyst is passed through a coolerand then through the second or nitric oxide removal catalyst. Thiscatalyst is temperature limited and hence the cooling step.

FIG. 2 has been presented to demonstrate the reason for the cooling ofthe exhaust stream before treatment with the nitric oxide destructioncatalyst. Temperatures not substantially above 900 F. should be employedin this catalyst since such temperature insures maximum nitric oxideconversion and avoids the catalyst degradation implicit in operation athigher temperatures.

separate curves which have been designated a, b, c and d;

Curve a depicts the conditions obtaining with the addition of noammonia. Curves b, c and d represent progressively larger additions ofammonia. Attention is invited to the fact that a highly satisfactoryelimination of nitric oxide is possible with no ammonia addition, butover a very narrow range of oxygen content in the exhaust stream. It hasbeen impossible to date to control the oxygen content of commercialengines to this degree of exactitude. The addition of small amounts ofammonia as shown by curves b, c and d greatly broadens the permissiblerange of oxygen contents of the exhaust stream which are consonant withsatisfactory catalytic reduction of nitric oxide to nitrogen.

FIG. 4 has been presented to demonstrate the exhaust chemistry of anormally operating vehicular engine and demonstrates the sensitivity ofthe exhaust composition to the air fuel ratio, particularly as thisratio departs in either direction from stoichiometric. A'considerationof this Figure and FIG. 3 demonstrates the utility of any inventionwhich will permit the catalytic reduction of nitric oxide under a widespread ofoxygen contents.

The copper oxide type of catalysts employable in this invention are wellknown and understood in the art and are furthermore very explicitlydescribed in the Griffings et al. patent. To this showing it is desiredto add the following specific copper oxide catalyst preparationprocedure which has been found to produce a very effective product.

A copper oxide catalyst is prepared by impregnating a support materialwith cupric nitrate solution. A solution containing 260 gm. of cupricnitrate, trihydrate dissolved in 2 liters of water is prepared.Following this, 1.5 liters of an activated alumina support containing 5percent silica is immersed in the solution for 2% hours. Afterdecantation, the impregnated support is dried at C. over night and thenis placed in a calcining oven at 565 C. for W; hours. After a secondhalf hour soak in the cupric nitrate solution followed by drying at 100C. over night, the catalyst is calcined for 3 hours at 590 C. in astream of flowing air.

The catalyst prepared in this manner contains about 5 percent copper inthe form of copper oxide on the support.

The noble metal catalysts required for this invention in cer tain of itsaspects are also well known to those skilled in the art. A particularlyeffective preparation is described in exam ple X of British Pat. 693,648which for convenience is reproduced below.

A catalyst similar to that prepared in example IX but having a lowdensity (0.45) was prepared in substantially the same manner asdescribed in example IX except as regards the ageing of thealumina-halogen spheres. The spheres in this example were aged in 1liter of hot Nujol" at a temperature of 9399 C. for about 2| hours, theNujol" was decanted from the spheres, and the spheres were then aged in1 liter of hot ammonium hydroxide solution (5 parts water and 1 partconcentrated ammonium hydroxide) at 9399 C. for about 24 hours. Thespheres were then washed, dried, calcined and composited" with platinumin substantially the same manner as described in example IX. Thecatalyst as prepared in this manner had a density of 0.45 (gm. per cc.)and contained 0.3 percent platinum. 0.33 percent combined fluorine and0.36 percent chlorine by weight. '(composited) as defined in ex ampleIX.

The calcined spheres were soaked in an aqueous solution ofchloroplatinic'acid in an amount to form a final catalyst co'htaining 03percent by weight of platinum. The solution was evaporated to drynessand the spheres were then dried at a temperature of 99 C. for 1 hour andfinally calcined at a tern perature gradually increasing to 500 C. for aperiod of 3 hours and then held at this temperature for 3 hours.

Ammonia is most conveniently and inexpensively obtained by thegasification of pressurized anhydrous liquid ammonia,

but the invention is by no means so limited Any compound which readilyreleases ammonia. or functions as its chemical equivalent may besubstituted for ammonia. Ammonium hydroxide, ammonium carbonate. urea.hexamethylene tetramme. hydrazine and ethylene diamine are amongcompounds which have been successfully substituted for ammonia gas inthis process.

Two catalysts were installed in series in the exhaust system from oneside of a V-8 engine as shown in FIG. 1. A water c. ooler was used tocool the exhaust before the second catalyst. The first catalyst. mainlyfor CO and hydrocarbon control, was the catalyst described inBritishPat. 693.648. The second catalyst for NO control was the platinumcatalyst described in that patent. The engine was operated on isooctanefuel (AIF=I4.8 at 2.000 rpm. and 15.5 in. Hg. absolute. Results obtainedat an exhaust temperature of 600 F. before the second catalyst and onthe addition of 0.05 cfm. gaseous ammonia to the exhaust at this pointare as follows:

CO, 01, C01,

NU, mole HC, mole mole p.p.rn. percent p.p.m. percent percent Before lslcatalyst. 1.508 1.55 335 0. 85 12.60 Before 2nd catalyst 665 0. 52 65 0.20 l3. 43 After 2nd catalysL 295 0. 29 45 0. 13. 60

the A/F ratio was I 5.l The results are as follows:

CO. )1, CO2 NU, Mole HG, mole mole. pp in. percent p.p.m. percentpercent l-lelo lz lst atalyst l 543 I ll) 255 U. 85 12. tit: Belon- Zud-ataly.-'t 590 U 52 45 0. 13.09 "t- 1 cataly t 95 ll 0 [3. 25

The following data illustrate the selectivity of the ammonia in removingNO even in the presence of l and 2 percent oxygen Inlet exhausttemperature was between 750 and 850 F gl) i C d 8 f zvu Inlet o -10 NO02 01 addt d (mole rrmoi'ed remaining removed remalnln pp-m Iptrrt't'tll). (ppm l .p.m.) lppm.) (p.p.rn. It; 2 1800 300 It, 500 6,0006,370 l t, 760 3%] 3, 500 3, 600 417 t LlBti l, 000 2, 500 4, 500

In these examples, the column d and fshuw the concentrations of NH and 0remaining in the exhaustfin each case the concentration of O isconsiderably greater tha he level of NO at the exit of the converter Theexc llent selectively of NH as a reducing agent is illustrated on thelast line of the table where 4|? p.p.m. NH, removed L080 p.p.m. NO inthe presence of excess 0, in the exhaust. The amount of O removed(column e) is considerably higher than the NO removed (column c) in eachcase. This is because CO H and hydrocarbons also react with 0, alongwith NH, to reduce the concentration ofO,.

A catalyst converter containing 400 in. of platinum catalyst wasinstalled in the exhaust system l2 inches from the exhaust manifoldflange on one side of a V-8 engine in a vehicle. The vehicle wasoperated at a Cruise 50 condition on a chassis dynamometer withnonleaded fuel. The following data were obtained at an A/F ratio of14.7, exhaust gas temperature of 1.180 F., with 0.1 cfm. of ammoniaadded to the exhaust prior to the catalyst:

C0, HC, 0:, 02,

NO, mole p.p.m. mole mole V p.p.m percent hexane percent percent Beforecatalyst. 3, 360 0. 29 0. 6 14.51 After catalyst 4. 000 0.07 10 The same400 in. converter was then located 5 feet further downstream in theexhaust pipe after a catalyst converter containing 1 17 in. of aplatinum catalyst. The arrangement was similar to that shown in FIG. 1except that water cooling was not used.

The following data were obtained at an A/F ratio of 14.9, exhausttemperature of 1,100 F. at the inlet to the 400 in. converter. with 0.]cfm. of ammonia again added to the ex- The data demonstrated theadvantage of two catalysts in series for the control of NO, CO andhydrocarbons. The 400 in. converter is much more effective in NO removalwhen preceded by the first catalyst due to lower temperatures, lower COconcentration, or higher oxygen level or a combination of these factors.

We claim:

1. The process of producing an internal combustion exhaust effluentcontaining greatly reduced percentages of carbon monoxide, unburnedhydrocarbons and oxides of nitrogen comprising operating the internalcombustion engine with the air to fuel weight ratio adjusted to a valuenot substantially lower than the stuichiometric value wherebyunacceptably high concentration of oxides of nitrogen are produced inthe exhaust stream. passing the exhaust stream directly from the engineto a first catalyst for the destruction at high tempera ture of asubstantial fraction of the carbon monoxide and un burned hydrocarbons.cooling the exhaust stream which has been impoverished with respect tocarbon monoxide and un burned hydrocarbons to a temperature notsubstantially in excess of 900 F; adding ammonia to the cooled stream inan amount to destroy l substantial portion of the oxides of nitrogendespite the presence of oxygen, and passing this cooled and ammoniatedexhaust stream over a second capable of causing the ammonia and oxidesof nitrogen to react to produce unobjectionable nitrogen and water.

2. The process recited in claim 1 in which the ammonia is at leastpartially replaced by a compound selected from the group consisting ofammonium hydroxide. ammonium carbonate, urea, hexame'thylene tetramine,hydrazine and ethylene diamine.

3. The process recited in claim I in which the catalysts are based upon--ither copperoxirle or the noble metals.

4. The pro t-'- recited in claim 1 in which the first men tionedcatalyst 15 based upon a noble metal and the second named catalyst uponcopper oxide.

2. The process recited in claim 1 in which the ammonia is at leastpartially replaced by a compound selected from the group consisting ofammonium hydroxide, ammonium carbonate, urea, hexamethylene tetramine,hydrazine and ethylene diamine.
 3. The process recited in claim 1 inwhich the catalysts are based upon either copper oxide or the noblemetals.
 4. The process recited in claim 1 in which the first mentionedcatalyst is based upon a noble metal and the second named catalyst uponcopper oxide.